GERD is caused by stomach acid repeatedly flowing back into the esophagus, most often because the muscular valve between your esophagus and stomach isn’t closing properly. This valve, called the lower esophageal sphincter (LES), normally opens to let food pass into your stomach and then tightens shut. When it relaxes at the wrong times or stays too loose, acid escapes upward. That backwash of acid is what produces the burning, chest pressure, and irritation that define GERD.
How the Valve Between Your Stomach and Esophagus Fails
The single most important cause of GERD is a pattern called transient lower esophageal sphincter relaxation. Your LES is supposed to relax briefly when you swallow, but in people with GERD it relaxes spontaneously, without any swallow triggering it. Each relaxation opens a brief window for acid to splash into the esophagus. Over the course of a day, those episodes add up.
Beyond these random relaxations, the junction where the esophagus meets the stomach can become physically looser over time. When the tissue around this junction stretches and loses its ability to snap back, it offers less resistance to acid flowing the wrong direction. Think of it like a door whose hinges have loosened: it no longer seals tightly, and it swings open with less force.
The Role of Hiatal Hernias
A hiatal hernia happens when part of your stomach pushes up through the opening in your diaphragm that the esophagus passes through. This displaces the LES from its normal position, removing the extra squeeze the diaphragm normally provides. In a study of 319 patients, 31% of those with a hiatal hernia had abnormally high acid exposure in the esophagus, compared to 18% of those without one. Having a hernia doesn’t guarantee you’ll develop GERD, but larger hernias make reflux more likely because the valve loses more of its structural support.
Excess Body Weight and Abdominal Pressure
Carrying extra weight, especially around your midsection, is one of the strongest modifiable risk factors for GERD. The mechanism is straightforward: more abdominal fat creates more pressure pushing against your stomach, which forces the LES open more easily and exposes the esophagus to acid. A CDC-affiliated study tracking thousands of adults found that people with obesity had a roughly 30% higher rate of developing GERD over a 14-year period compared to those at a normal weight. Even being overweight without reaching obesity raised the risk meaningfully. This pressure effect also explains why GERD symptoms often worsen after large meals or when you lie down, both of which increase the force pushing stomach contents upward.
Pregnancy and Hormonal Changes
Up to two-thirds of pregnant women experience heartburn, and the cause is twofold. Rising progesterone levels relax smooth muscle throughout the body, including the LES. A relaxed sphincter lets acid escape more easily. At the same time, progesterone slows the rate at which your stomach empties, keeping food and acid sitting there longer. As pregnancy progresses, the growing uterus adds physical pressure on the stomach from below, compounding the hormonal effect. For most women, GERD symptoms resolve after delivery once hormone levels normalize and the abdominal pressure drops.
Smoking and Saliva
Smoking contributes to GERD through a less obvious pathway: it reduces your body’s ability to clear acid from the esophagus. Normally, after a small reflux episode, a swallow or two of saliva neutralizes the residual acid clinging to the esophageal lining. Saliva contains bicarbonate, a natural acid buffer. Cigarette smoking suppresses salivary gland output, including bicarbonate production. The result is that acid sits on the esophageal tissue longer after each reflux event, increasing the chance of inflammation and damage. Studies comparing smokers and nonsmokers found no real difference in how well the esophagus physically pushes fluid downward. The problem isn’t the muscle contractions; it’s the chemical cleanup step that smoking impairs.
Medications That Weaken the Valve
Several common drug classes can relax the LES or worsen acid exposure. Blood pressure medications like calcium channel blockers and nitrates directly reduce sphincter tone. Certain antidepressants, sedatives (including benzodiazepines), opioid painkillers, and medications for overactive bladder all have similar effects. Even progesterone supplements can contribute, which partly explains why hormone-based birth control occasionally triggers reflux in women who never had it before. If your GERD started or worsened around the time you began a new medication, that connection is worth exploring with whoever prescribed it.
Slow Stomach Emptying
When your stomach takes longer than normal to push food into the small intestine, acid and partially digested food linger, raising the odds of reflux. In one study of GERD patients, about a third showed significantly delayed stomach emptying at the two-hour mark. Interestingly, the delay didn’t always produce distinctive symptoms. Patients with slow emptying reported the same mix of regurgitation and difficulty swallowing as those with normal emptying, which means you can’t tell from symptoms alone whether this is part of your problem. Conditions like diabetes and certain neurological disorders can slow gastric motility, as can some of the same medications that relax the LES.
Foods, Habits, and Timing
Certain foods don’t cause GERD in a structural sense, but they can trigger or worsen individual episodes. Fatty foods slow stomach emptying. Chocolate, peppermint, and alcohol relax the LES. Acidic foods like tomatoes and citrus don’t increase the amount of reflux, but they irritate tissue that’s already inflamed, making symptoms feel worse. Coffee stimulates acid secretion in some people.
Eating large meals and lying down within two to three hours of eating are among the most consistent behavioral triggers. Gravity is one of your best defenses against reflux. When you’re upright, acid has to travel against gravity to reach your esophagus. When you recline with a full stomach, that barrier disappears. Eating smaller meals and staying upright afterward reduces both the frequency and severity of reflux episodes without any medication at all.
Why Some People Get GERD and Others Don’t
GERD rarely comes down to a single cause. It’s usually a combination: a slightly weak sphincter, a modest hiatal hernia, some extra abdominal weight, a medication that relaxes smooth muscle, and a habit of eating late at night. Each factor alone might not produce symptoms, but together they push the system past its threshold. This is also why treatment often works best when it targets multiple contributors at once, such as losing weight, adjusting meal timing, and elevating the head of the bed, rather than relying on a single change.
Some people also have an esophageal lining that’s more sensitive to acid than average. Two people can have identical amounts of reflux, yet one develops pain and inflammation while the other notices nothing. This variation in tissue sensitivity helps explain why GERD severity doesn’t always match the degree of sphincter dysfunction on testing.